The present invention relates to methods and devices for providing considerable fluoroscopic visibility and high electrical conductivity without the undesired effects of secondary local dose enhancement of x-ray radiation.
In co-pending U.S. patent application Ser. No. 09/081,954 ("the '954 application") the applicants herein disclose methods and systems for providing differential local dose enhancement of incident ionizing radiation for purposes of inhibiting tissue cell proliferation, which may include suppression of neointimal hyperplasia that characterizes restenosis such as that following coronary or carotid artery angioplasty or trauma to other body vessels, ducts or tracts, or destruction of malignant tumors.
An x-ray source can generate a beam of radiation with an energy content in a range from 10 KeV (000's of electron Volts) to 400 MeV. If a radiation beam of several hundred KeV contacts a metal surface, it was assumed in the past that the dosage of x-ray radiation is amplified by a factor of from 1.8 to 2.5 at the metal surface owing to backscatter radiation, which is attributable to a photo-multiplier effect. Amplification by such a factor had not been deemed either useful or dangerous. The applicants herein discovered in their research, however, that at radiation energy levels below about 400 KeV the amplification factor from backscatter from the metallic surface is inversely proportional to the KeV level of the radiation and, in the vicinity of the metal structures, can be 100 times the radiation dosage that exists in the same location in the absence of such structures. An increasingly higher amplification factor is experienced at the metal surface for increasingly lower radiation intensity, with a maximum factor of about 200.times. occurring for a radiation level of 40 KeV. The applicants further found that the range and intensity of the backscatter radiation depends on the physical characteristics of the metal surface, particularly on the atomic number of the metal, expressed as Z, in the periodic table of elements. A maximum backscatter radiation effect occurs at Z=60, corresponding to the atomic number for the element iodine, and a reduction in backscatter amplification occurs for metals with atomic numbers lower or higher than 60. The noble metals such as gold, platinum and iridium, for example, are at about Z=80, which yields an amplification factor of about 100.
As disclosed in the co-pending '954 application, the amplification effect enables a closely controlled local dosage enhancement of impinging radiation for therapeutic applications involving tissue cell proliferation found in vascular restenosis and solid tumors. A radiation dosage of 20 Gray (Gy) is effective to inhibit proliferation of smooth muscle cells in a carotid artery following angioplasty. However, by implanting (or employing a previous implant of) a metallic stent which is used to hold open the lumen of the artery at the angioplasty site, and irradiating the adjacent tissue with the stent in place, the amplification from backscatter radiation can enhance the dosage to the tissue of interest to allow use of radiation of lower intensity. Moreover, if the stent has a coating of a noble metal, a beam of x-ray radiation of about 40 KeV incident on that metal surface can deliver a dosage at that location of approximately 100.times. the dosage that would be delivered without the presence of the metal surface despite even use of a beam of considerably higher level of energy content. Thus, a system and method which would produce a dosage of only 0.20 Gy to tissue at a site of interest can be made to yield an effective dosage of 20 Gy by use of the principles of that invention.
Despite these advantages which are attainable by beneficial use of this secondary local dose enhancement, and the improvements thereof provided by the invention of the co-pending application, there remain situations in which such secondary enhancement attributable to the presence of a metallic surface in an environment in which a region of the body is being irradiated with ionizing radiation is highly undesirable.
It is a principal aim of the present invention to provide methods and means of avoiding unwanted amplification of incident radiation where metal surfaces are present.
Various routine investigations or examinations of a patient for diagnostic purposes, such as in the field of electrophysiology, require the use of a number of catheters which are inserted into the patient's body. By way of example, ablation procedures call for the placement of mapping catheters in firm contact with the heart so as to evoke responses of the heart to signals transmitted from an external electrical physiological work station, and to detect intrinsic electrical signals of cardiac activity which are recorded at the work station. Metallic surfaces of electrodes on the catheters serve not only to enable transmission and reception of these signals, but to enhance the visibility of the catheters under x-ray fluoroscopy for precise placement. The higher the atomic number of the metal used for the electrodes, the more enhanced is the fluoroscopic visibility. Accordingly, the noble metals are typically used, if only as a surface coating, for this purpose.
As pointed out above with reference to co-pending '954 application, however, even relatively low level x-ray radiation used for fluoroscopy can be significantly multiplied to deliver harmful dosages of ionizing radiation to body tissue in the immediate vicinity of the metallic surfaces. In research performed by one of the applicants herein, measurement of the physical and biological interface dose effects in tissue attributable to x-ray released secondary radiation from metallic gold surfaces indicates that the number of oncogenic transformations (i.e., gene-induced transformation of cells from normal proliferation to a potentially malignant state) increases by a factor of 20 at dose levels generally considered to be safe--about 100 microGray (mGy). This increase in transformed oncogenic cells depends on the presence of metallic surfaces and their effect in amplifying radiation dosage, and is clearly undesirable.
Accordingly, a more specific objective of the invention is to provide a metallic catheter or other medical investigative device to be placed in the body for examining, exploratory, diagnostic or related purposes--even therapeutic, where the amplification effect of radiation dosage disclosed in the above co-pending application is not sought--, and in which the metallic surface renders the device adequately visible under fluoroscopy, but which is implemented with safeguards to avoid the oncogenic effect attributable to incidence of x-ray radiation on the metallic surface.